As is known, the configurations of an automobile gear transmission are characterized by rather consolidated schemes, which envisage use of: a primary shaft, on which the driving gears are fitted, either idle or fixed; a secondary shaft, on which the driven gears are fitted, again either idle or fixed; a mechanism for synchronization and engagement between the idle gears and the shaft on which these are fitted; a pair of gears for final reduction of the transmission ratio at output from the secondary shaft; and, finally, a differential, which supplies the final output of the motion to two axle shafts.
The above arrangement is accompanied by a control device, which constitutes an interface between the driver and the synchronization and engagement mechanism. In particular, the transmission ratios or gears are engaged selectively by operating a gear lever in the passenger compartment. Said lever actuates a finger-shaped control member, which in turn actuates a series of gearshift forks, which are movable in a direction parallel to the axes of the primary and secondary shafts. The gearshift forks are arranged on the outside of said shafts, correspond to respective ranges of the gears (for example, the range of the first and second gears, the range of the third and fourth gears, and the range of the fifth and reverse gears), and are associated to respective parallel control seats facing one another, engaged by the finger member. The finger member is movable along a selection path that freely traverses the control seats for selecting one of said control seats and, hence, a range of the gears (first and second, or else third and fourth, or else fifth and reverse) and along an engagement path parallel to the planes of lie of the control seats for displacing the seat selected and engaging one of the two gears of the corresponding range, said finger member shifting in opposite directions starting from a central position, to which there corresponds an idle or “neutral” condition. In general, one of the two paths of the finger member is rectilinear, whilst the other is defined by a rotation.
In known solutions, each gearshift fork engages a corresponding sleeve of the synchronization and engagement mechanism. For each pair of idle gears associated to the same range and fitted on the same shaft (whether primary or secondary), the synchronization and engagement mechanism is normally provided with: a hub fitted externally on the shaft; a pair of driving rings arranged on opposite axial sides of the hub, each of which is angularly fixed with respect to a corresponding gear and which have respective external toothings; and a pair of rings for synchronizing the motion that are able to activate the driving rings selectively.
The synchronizing rings are each set axially between the hub and the corresponding driving ring, have respective conical friction surfaces designed to co-operate, in use, with corresponding conjugated friction surfaces carried by the driving rings, and are provided with respective external toothings.
The sleeve is located on the outside of the hub and is rendered angularly fixed with respect to the hub itself by means of an internal toothing of its own. The sleeve can slide axially on opposite sides of the hub under the action of the corresponding gearshift fork. During the axial travel of the sleeve in one direction or else in the other, the internal toothing of the sleeve engages first the external toothing of the synchronizing ring and then the external toothing of the driving ring, when the relative speed of the latter with respect to the synchronizing ring goes to zero as a result of the contact between the respective conjugated friction surfaces.
The synchronization and engagement mechanism just described, albeit satisfactory from the functional standpoint, as a whole requires a space that is relatively large, in particular in a direction parallel to the axes of the primary and secondary shafts.
Indeed, the engagement between the sleeve and the gearshift fork, and the travel of the sleeve itself in opposite directions with respect to the central idle position impose the need for a minimum axial distance that is relatively large between the two gears associated to the same range.
Another solution is disclosed in WO2005/036007A1, where a sliding sleeve is rotationally fixed between two gear wheels so that it can be displaced axially on a gearbox shaft. Said gearbox shaft is configured as a hollow shaft. A gear-shift rod is mounted in the gearbox shaft so that it can be axially displaced. The axial shift displacements of the gear-shift rod are transmitted to the sliding sleeve by means of radial connection elements. Synchronisation rings are located between the sliding sleeve and the gear wheels in order to adapt the speeds of the sliding sleeve and the gear wheel that is to be shifted to one another during the shift operation.
Also this solution requires a space that is relatively large.